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Abstract

Nanostructures composited of vertical rutile TiO2 nanorod arrays and Sb2S3 nanoparticles were prepared on an F:SnO2 conductive glass by hydrothermal method and successive ionic layer adsorption and
reaction method at low temperature. Sb2S3-sensitized TiO2 nanorod solar cells were assembled using the Sb2S3-TiO2 nanostructure as the photoanode and a polysulfide solution as an electrolyte. Annealing
effects on the optical and photovoltaic properties of Sb2S3-TiO2 nanostructure were studied systematically. As the annealing temperatures increased,
a regular red shift of the bandgap of Sb2S3 nanoparticles was observed, where the bandgap decreased from 2.25 to 1.73 eV. At
the same time, the photovoltaic conversion efficiency for the nanostructured solar
cells increased from 0.46% up to 1.47% as a consequence of the annealing effect. This
improvement can be explained by considering the changes in the morphology, the crystalline
quality, and the optical properties caused by the annealing treatment.

Keywords:

TiO2; Sb2S3; Nanorod; Solar cells; Annealing effect

Background

Dye-sensitized solar cells (DSSCs) pioneered by O'Regan and Grätzel have been intensively
investigated as a promising photovoltaic cell all over the world [1-5]. Until now, photovoltaic conversion efficiency of up to 11% has been reported for
DSSCs in the laboratory [6]. Although the conversion efficiency is impressive, the expense of the dye required
to sensitize the solar cell is still not feasible for practical applications. Therefore,
it is critical to tailor the materials to be not only cost effective but also long
lasting. Recently, the utilization of narrow-bandgap semiconductors as a light-absorbing
material, in place of conventional dye molecules, has drawn much attention. Inorganic
semiconductors have several advantages over conventional dyes: (1) The bandgap of
semiconductor nanoparticles can be easily tuned by size over a wide range to match
the solar spectrum. (2) Their large intrinsic dipole moments can lead to rapid charge
separation and large extinction coefficient, which is known to reduce the dark current
and increase the overall efficiency. (3) In addition, semiconductor sensitizers provide
new chances to utilize hot electrons to generate multiple charge carriers with a single
photon. These properties make such inorganic narrow-bandgap semiconductors extremely
attractive as materials for photovoltaic applications.

Recently, a range of nano-sized semiconductors has been investigated in photovoltaic
applications including CdS [7-9], CdSe [10-13], Ag2S [14], In2S3[15], PbS [16], Sb2S3[17], Cu2O [18], as well as III-VI quantum ring [19]. Among these narrow-bandgap semiconductors, Sb2S3 has shown much promise as an impressive sensitizer due to its reasonable bandgap
of about 1.7 eV, exhibiting a strong absorption of the solar spectrum. The use of
Sb2S3 nanoparticles, which may produce more than one electron–hole pair per single absorbed
photon (also known as multiple exciton generation), is a promising solution to enhance
power conversion efficiency. Furthermore, the creation of a type-II heterojunction
by growing Sb2S3 nanoparticles on the TiO2 surface greatly enhances charge separation. All of these effects are known to increase
the exciton concentration, lifetime of hot electrons, and therefore, the performance
of sensitized solar cells. Limited research has previously been carried out with Sb2S3-TiO2 nanostructure for solar cell applications [20-22]. A remarkable performance was obtained in both liquid cell configuration and solid
configuration. These findings were based on the use of porous nanocrystalline TiO2 particles; however, very little research has been conducted using single-crystalline
TiO2 nanorod arrays. Compared with conventional porous polycrystalline TiO2 films, single-crystalline TiO2 nanorods grown directly on transparent conductive oxide electrodes provide an ideal
alternative solution by avoiding particle-to-particle hopping that occurs in polycrystalline
films, thereby increasing the photocurrent efficiency. Further enhancements in solid
Sb2S3-sensitized solar cells demand a deeper understanding of the main parameters determining
photoelectric behavior while also requiring additional research and insight into the
electrical transporting process in these nanostructures.

In our present research study, Sb2S3 semiconductor nanoparticles and single-crystalline rutile TiO2 nanorod arrays were combined to perform as a photoanode for a practical nanostructured
solar cell (as depicted in Figure 1). The annealing effect on the photovoltaic performance and optical property of Sb2S3-TiO2 nanostructures was studied systematically, and the optimal temperature of 300°C was
confirmed. After annealing, apparent changes of morphological, optical, and photovoltaic
properties were observed. The photovoltaic conversion efficiency of solar cell assembled
using annealed Sb2S3-TiO2 nanostructure demonstrated a significant increase of 219%, compared with that based
on as-made Sb2S3-TiO2 nanostructure.

Methods

TiO2 nanorod arrays were grown directly on fluorine-doped tin oxide (FTO)-coated glass
using the following hydrothermal methods: 50 mL of deionized water was mixed with
40 mL of concentrated hydrochloric acid. After stirring at ambient temperature for
5 min, 400 μL of titanium tetrachloride was added to the mixture. The feedstock, prepared
as previously described, was injected into a stainless steel autoclave with a Teflon
lining. The FTO substrates were ultrasonically cleaned for 10 min in a mixed solution
of deionized water, acetone, and 2-propanol with volume ratios of 1:1:1 and were placed
at an angle against the Teflon liner wall with the conducting side facing down. The
hydrothermal synthesis was performed by placing the autoclave in an oven and keeping
it at 180°C for 2 h. After synthesis, the autoclave was cooled to room temperature
under flowing water, and the FTO substrates were taken out, washed extensively with
deionized water, and dried in open air.

Successive ionic layer adsorption and reaction (SILAR) method was used to prepare
Sb2S3 semiconductor nanoparticles. In a typical SILAR cycle, the F:SnO2 conductive glass, pre-grown with TiO2 nanorod arrays, was dipped into the 0.1 M antimonic chloride ethanol solution for
5 min at 50°C. Next, the F:SnO2 conductive glass was rinsed with ethanol and then dipped in 0.2 M sodium thiosulfate
solution for 5 min at 80°C and finally rinsed in water. This entire SILAR process
was repeated for 10 cycles. After the SILAR process, samples were annealed in N2 flow at varied temperatures from 100°C to 400°C for 30 min. After annealing, a color
change was noted in the Sb2S3-TiO2 nanostructured samples, which were orange before annealing and gradually turned blackish
as the annealing temperature increased.

Solar cell assembly and performance measurement

Solar cells were assembled using a Sb2S3-TiO2 nanostructure as the photoanode. Pt counter electrodes were prepared by depositing
an approximately 20-nm Pt film on FTO glass using magnetron sputtering. A 60-μm-thick
sealing material (SX-1170-60, Solaronix SA, Aubonne, Switzerland) with a 3 × 3 mm
aperture was pasted onto the Pt counter electrodes. The Pt counter electrode and the
Sb2S3-TiO2 sample were sandwiched and sealed with the conductive sides facing inward. A polysulfide
electrolyte was injected into the space between the two electrodes. The polysulfide
electrolyte was composed of 0.1 M sulfur, 1 M Na2S, and 0.1 M NaOH which were dissolved in distilled water and stirred at 80°C for
2 h.

A solar simulator (Model 94022A, Newport, OH, USA) with an AM1.5 filter was used to
illuminate the working solar cell at light intensity of one sun illumination (100
mW/cm2). A source meter (2400, Keithley Instruments Inc., Cleveland, OH, USA) was used for
electrical characterization during the measurements. The measurements were carried
out using a calibrated OSI standard silicon solar photodiode.

Results and discussion

Morphology and crystal structure of Sb2S3-TiO2 nanostructure

The morphology of the rutile TiO2 nanorod arrays is shown in Figure 2a. The SEM images clearly show that the entire surface of the FTO glass substrate
was uniformly covered with ordered TiO2 nanorods, and the nanorods were tetragonal in shape with square top facets. This
nanorod array presented an easily accessed open structure for Sb2S3 deposition and a higher hole transferring speed for the whole solar cell. No significant
changes in nanorod array morphology were observed after annealing at 400°C. As-synthesized
Sb2S3-TiO2 nanostructure is shown in Figure2b, indicating a combination of the Sb2S3 nanoparticles and TiO2 nanorods. The Sb2S3-TiO2 nanostructure after annealing at 300°C for 30 min is shown in Figure 2c. Compared to the CdS-TiO2 nanostructure, in which 5-to 10-nm CdS nanoparticles distributed uniformly on the
TiO2 nanorod [9], the as-deposited Sb2S3 particles differed with a larger diameter of approximately 50 nm and often covered
several TiO2 nanorods. This structural phenomenon was observed much more so in the annealed sample,
where at least some melting of the low melting point (550°C) Sb2S3 clearly occurred. After the annealing treatment, the size of Sb2S3 particles increased, which enabled the Sb2S3 particles to closely contact the TiO2 nanorod surface. This solid connection between Sb2S3 nanoparticles and the TiO2 nanorods was beneficial to the charge separation and improved the overall properties
of the sensitized solar cells.

X-ray diffraction (XRD) patterns of the bare TiO2 nanorod array, the as-synthesized Sb2S3-TiO2 nanostructure, and the annealed nanostructure are shown in Figure 3. Note in Figure 3a that the TiO2 nanorod arrays grown on the FTO-coated glass substrates had a tetragonal rutile structure
(JCPDS no. 02–0494), which may be attributed to the small lattice mismatch between
FTO and rutile. The as-synthesized Sb2S3-TiO2 nanostructure exhibited a weak diffraction peak (Figure 3b) at 2θ = 28.7°, corresponding to the (230) plane of orthorhombic Sb2S3. As the annealing temperature increased, more diffraction peaks were observed, and
the peaks became more distinct at the same time. Figure 3c shows the XRD pattern of the nanostructure annealed at less than 300°C. All of the
reflections were indexed to an orthorhombic phase of Sb2S3 (JCPDS no. c-74-1046) [23]. The shape of the diffraction peaks indicates that the product was well crystallized.

Optical property of the Sb2S3-TiO2 nanostructures

The UV-visible absorption spectra of Sb2S3-TiO2 nanostructure samples are shown in Figure 4. An optical bandgap of 2.25 eV is estimated for the as-synthesized Sb2S3 nanoparticles from the absorption spectra, which exhibits obvious blueshift compared
with the value of bulk Sb2S3. After being annealed at 100°C, 200°C, and 300°C for 30 min, the bandgap of Sb2S3 nanoparticles was red shifted to 2.19 eV (565 nm), 2.13 eV (583 nm), and 1.73 eV
(716 nm), respectively. When annealed at 400°C, the absorption spectra deteriorated,
which may be attributed to the oxidation as well as the evaporation of the Sb2S3 nanoparticles. The Sb2S3-TiO2 nanostructure annealed at 300°C shows an enhanced absorption in the visible range,
which is of great importance for solar cell applications and will result in higher
power conversion efficiency. As shown by the XRD patterns and SEM images, this red
shift in the annealed samples may be explained by the annealing-induced increase in
particle size at the elevated temperatures. The annealing effect on the optical absorption
spectra of bare TiO2 nanorod arrays was also studied (not included here). No obvious difference was found
between the samples with and without annealing treatment. This result suggests that
although annealing changes the morphology and crystallinity of Sb2S3 nanoparticles, it does not significantly affect the optical property of TiO2 nanorod arrays.

Photovoltaic performance of the solar cell based on Sb2S3-TiO2 nanostructure

The photocurrent-voltage (I-V) performances of the solar cells assembled using Sb2S3-TiO2 nanostructures annealed under different temperatures are shown in Figure 5. The I-V curves of the samples were measured under one sun illumination (AM1.5, 100 mW/cm2). Compared with the solar cell based on as-grown Sb2S3-TiO2 nanostructure, the solar cell performances correspondingly improved as the annealing
temperatures increased from 100°C to 300°C. The open-circuit voltage (Voc) improved from 0.3 up to 0.39 V, and the short-circuit current density (Jsc) improved from 6.2 up to 12.1 mA/cm2. A power conversion efficiency of 1.47% for the sample with annealing treatment was
obtained, indicating an increase of 219% (as compared to the 0.46% for the as-grown
sample) as a consequence of the annealing treatment. The photovoltaic performance
of annealed Sb2S3-TiO2 nanostructured solar cell under 400°C deteriorated, which coincides with the absorption
spectrum. Detailed parameters of the solar cells extracted from the I-V characteristics are listed in Table 1.

This significant improvement of the photovoltaic performance obtained for annealed
Sb2S3-TiO2 nanostructured solar cells is explained by the following reasons: (1) An enhanced
absorption of sunlight caused by the red shift of the bandgap will result in an enhanced
current density. (2) Increase of Sb2S3 grain size by annealing will reduce the particle-to-particle hopping of the photo-induced
carrier. This hopping may occur in an as-grown nanostructure with Sb2S3 nanoparticles. (3) Improvement of crystal quality of the Sb2S3 nanoparticles by annealing treatment will decrease the internal defects, which can
reduce the recombination of photoexcited carriers and result in higher power conversion
efficiency. (4) Good contact between the Sb2S3 nanoparticles and the TiO2 nanorod is formed as a result of high-temperature annealing. Such a superior interface
between TiO2 and nanoparticles can inhibit the interfacial recombination of the injected electrons
from TiO2 to the electrolyte, which is also responsible for its higher efficiency.

Our findings suggest the possible use of 3D nanostructure material grown by a facile
hydrothermal method for sensitized solar cell studies. The drawback of this type of
solar cell is a rather poor fill factor, which limits the energy conversion efficiency.
This low fill factor may be ascribed to the lower hole recovery rate of the polysulfide
electrolyte, which leads to a higher probability for charge recombination [24]. To further improve the efficiency of these nanorod array solar cells, we advise
that a new hole transport medium with suitable redox potential and low electron recombination
at the semiconductor and electrolyte interface should be developed. Moreover, as reported
by Soel et al., other contributions such as the counter electrode material may also
influence the fill factor [25].

Conclusions

With a facile hydrothermal method, the single-crystalline TiO2 nanorod arrays were successfully grown on fluorine-doped tin oxide glass. Next, Sb2S3 nanoparticles were deposited by successive ionic layer adsorption and reaction method
to form a Sb2S3-TiO2 nanostructure for solar cell applications. Annealing treatment was conducted under
varied temperatures, and the optimal annealing temperature of 300°C was obtained.
Obvious enhancement in visible light absorption was observed for the annealed samples.
The photovoltaic performance for solar cells based on annealed Sb2S3-TiO2 nanostructure shows an increase of up to 219% in power conversion efficiency.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

YL carried out the preparation of Sb2S3-TiO2 nanostructured solar cells and drafted the manuscript. LW conducted the optical absorption
spectra and the I-V measurements. RZ carried out the preparation of TiO2 nanorod arrays and the XRD measurements. YC carried out the SEM characterization
and supervised the work. LM and JJ analyzed the results and finalized the manuscript.
All authors read and approved the final manuscript.

Acknowledgments

This work was supported by the National Key Basic Research Program of China (2013CB922303,
2010CB833103), the National Natural Science Foundation of China (60976073, 11274201,
51231007), the 111 Project (B13029), the National Found for Fostering Talents of Basic
Science (J1103212), and the Foundation for Outstanding Young Scientist in Shandong
Province (BS2010CL036).